When I send mail with attachments to people using MS Outlook, they see an extra
attachment <blurp>.ATT included in the file. They cannot open this file, and
this causes quite some confusion. Apparently its my signature that is contained 
in this attachment (as it does not show up in elsewhere in their message pane).

What is the problem and how can it be solved?

I've included a message-as-textfile example as attachment.